This application relates generally to characterizing the positioning of recording heads over tracks divided into physical sectors in a disc drive, and more particularly to a position sensing system utilizing small servo sectors and side-by-side read/write (R/W) recording heads designed to, while minimally affecting performance, predict write error occurrences by sampling and analyzing recording head position data acquired from the small servo sector and side-by-side R/W recording head configuration.
Disc drives are data storage devices that store digital data in magnetic form on a rotating storage medium called a disc. Modern disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Each surface of a disc is divided into several thousand tracks that are tightly packed concentric circles similar in layout to the annual growth rings of a tree. The tracks are typically numbered starting from zero at the track located outermost the disc and increasing for tracks located closer to the center of the disc. Each track is further broken down into data sectors and servo bursts. A data sector is normally the smallest individually addressable unit of surface area in which to store information on a disc in a disc drive and typically holds 512 bytes of information plus a few additional bytes for internal drive control and error detection and correction functions. This organization of data allows for easy access to any part of the disc surface.
A servo burst, also known as a servo sector, is a particular magnetic signature on a track that facilitates positioning of read/write (R/W) transducers or heads accurately over the tracks. Servo sectors cross track boundaries, and can be envisioned essentially as radial spokes of a wheel. The conventional format of a servo sector is as follows. The first element of a servo sector is the variable frequency oscillator (VFO) field. This is also often referred to as an AGC field. Typically, the VFO field accounts for one half the size, or length, of a servo sector. The purpose of the VFO field is to generate an on-the-fly frequency by which subsequent servo sector data can be read. Following the VFO field is typically a servo address mark (SAM), This is also often referred to as a servo timing mark, and is typically approximately 10 bits of data. The purpose of the SAM is to indicate the starting point of the servo sector data. Following the SAM is the servo sector data. This data contains track address information, describing which track the head is on. Finally, after the servo sector data is the PES (position error signal). This is also oftentimes referred to as the vernier position signal. The purpose of the PES is to provide a means for the control system to determine the center of the track for proper head positioning.
Generally, each of the multiple discs in a disc drive has associated with it two heads (one adjacent the top surface of the disc, and another adjacent the bottom) for reading and writing data to a sector. A typical disc drive has two or three discs. This usually means there are four or six heads in a disc drive carried by a set of actuator arms. Data is accessed by moving the heads from the inner to outer part of the disc (and vice-versa) driven by an actuator assembly. The heads that access sectors on discs are locked together on the actuator assembly. For this reason, all the heads move in and out together and are always physically located at the same track number (e.g., it is impossible to have one head at track 0 and another at track 500). Because all the heads move together, each of the tracks on all discs is known as a cylinder for reasons that these tracks form a cylinder since they are equal-sized circles stacked one on top of the other in space. So, for example, if a disc drive has four discs, it would normally have eight heads, and a cylinder number 680 would be made up of a set of eight tracks, one per disc surface, at track number 680. Thus, for most purposes, there is not much difference between tracks and cylinders since a cylinder is basically a set of all tracks whereat all the heads are currently located.
One of the heads must first be positioned over the correct location of a sector on the disc in order to access (i.e., read from or write to) the sector. This requires the heads to move to the correct track and then wait for the correct sector to pass under the appropriate head. Moving the heads to the correct track is referred to as a seek. Once a seek has finished and while the disc rotates to a correct sector, the servo mechanism continuously interprets servo sector information from the track to ensure the head remains positioned correctly. Essentially, servo sectors, also known as servo bursts, aid in steering the head over the track.
FIG. 3 is a schematic representation of a conventional servo sector 200 recorded on a disc in a sectored servo control system scheme. Five typical concentric tracks 202, 204, 206, 208, and 210 are pictured sequentially in the vertical direction. The horizontal lines in FIG. 3 indicate the track boundaries. The servo sector format is interpreted from left to right partitioned bit-wise. The first field is simply a 2 bit gap 212. The gap is used to indicate separation of the servo sector from the previous data sector. The second field is the variable frequency oscillator (VFO) field 214. The VFO field 214 is typically equal to half the length of the entire servo sector 200. The purpose of the VFO field 214 is to generate a frequency for the data sampling rate to lock onto in order to precisely time the reading of subsequent servo sector information bits. Following the VFO field 214 is a servo timing mark, or servo address mark (SAM) 216. The SAM 216 is typically 10 bits in length and indicates the start of subsequent servo sector information bits. Following the SAM 216, there are several bits of information indicating the track 152 address according current precise location on the disc 108. In the illustrated format, there are 18 bits of track address information 218. Following the track address information 218, there are 12 bits of vernier position error signal normal (PES_N) 220, followed by 12 bits of vernier position error signal quadrature (PES_Q) 222. The PES signals 220, 222, are used to steer the recording head affixed to the actuator assembly over the center of the track, e.g. track 204. Finally, there are 2 additional bits of gap 224.
Recording transducers or heads consist of two elements: a read element, or reader, and a write element, or writer. Conventionally, the reader and writer element are positioned sequentially in a recording head. This is also referred to as a piggyback configuration. FIG. 5 is a schematic bottom view of a convention piggyback recording head 300 and a corresponding track 204 on a disc. The reader and writer of the recording head 300 are oriented such that they fly over the same track. The reader and writer each has a corresponding read gap 304 and write gap 302. Due to the piggyback configuration, a distance 306, typically 5 xcexcm, separates these gaps. Also pictured is a track 204 with data sectors 226 surrounding a servo sector 200. During operation, the disc rotates such that the track 204 passes under the piggy-back recording head 300 from right to left in the direction indicated by the arrow 308. A piggy-back recording head 300 can only read or write at any given time, and since a piggy-back recording head 300 must first read a servo sector 200 before it may write the subsequent data sector 226, a gap 306 equal to the distance between the read gap 304 and write gap 302 also exists on the disc media as shown.
Thus, as the disc passes under the head, the reader reads servo information until a data sector is to be read or written. When a data sector is to be written, the disc rotates until the servo sector just passes under the read head. At this point, the writer may begin to write data. Due to the gap between the reader and the writer, there is therefore an amount of wasted space on the disc after each servo sector equal to the gap 306. This wasted space is of no concern in conventional disc drives due to the relatively small number of servo sectors spaced around the disc surface.
As with any data storage and retrieval scheme, data integrity is critical. The reliability of a hard disc drive is desired to be less than 1 data loss in one trillion data accesses, or less than 1 read failure in 1012 attempted read operations. Often, for various reasons such as defective media, improper head positioning, extraneous particles between the head and media, or marginally functioning components, disc drives may sometimes record or read data incorrectly to or from the disc. A predominant cause for failed data reads is that the initial write operation of a particular sector was unsuccessful. Unsuccessful writes are usually attributed to improper positioning of the recording head over the media which is caused by either an extraneous particle between the head and media, or by physical shocks imposed on the disc drive itself, causing the servo mechanism to jolt.
External shocks experienced by the disc drive can cause write failures, and the failures can be divided into three categories. In once case, a rotational shock essentially bumps the head in the circumferential direction, lengthening or shortening the periodic spacing of bits being written. This can result in overwriting an adjacent sector on the same track, and is referred to as a xe2x80x9cadjacent sector overwritexe2x80x9d. In a second case, a rotational shock might bump the recording head in a radial direction, causing the head to be positioned off track. This can result in writing to an incorrect sector on an incorrect track, and is referred to as a xe2x80x9cwrite faultxe2x80x9d. In a third case, a translational shock or asperity might cause the recording head to jump from the disc resulting in too great a distance between the head and the disc. This can result in failing to correctly record data to a sector, and is referred to as a xe2x80x9cskip writexe2x80x9d.
Conventionally, minimizing the number of write errors in a disc drive is accomplished by minimizing track mis-registration. Essentially, track mis-registration, also referred to as RMS servo error, is a characterization of how precisely a recording head is positioned over a track. A greater RMS servo error would indicate a more improperly positioned head, and a higher probability of incorrectly recorded data during a write operation. To this end, a disc drive servo system is conventionally designed with a particular amount of tolerance, directed to minimize RMS servo error. The problem with this method is that the disc drive is not actually designed to detect and correct actual particular write failures. Rather, it is designed on a statistical basis to withstand certain shocks and anomalies, utilizing a range of tolerance for error. Moreover, as disc drive designs continue to evolve, the number of tracks on a disc, or track density, will continue to increase. As track density increases, the distance between tracks, or track pitch, will decrease. As track pitches continue to become smaller, for example to 170 xcexcm, the conventional servo system that minimizes RMS servo error will no longer be able to statistically withstand common shocks and anomalies. A jolt may now cause a head to move halfway off track. In the future, the same jolt may cause a head to move several tracks off. Consequently, write failures and subsequent read failures will increase rendering the conventional servo system unusable.
Accordingly there is a need for a servo position sensing system to actually ensure the recording head is positioned over the correct sector and track while minimally affecting a disc drive""s storage capacity.
Against this backdrop embodiments of the present invention have been developed. Embodiments of the present invention essentially involve a unique recording head facilitating a system to take frequent measurements of the position of the recording head with respect to the disc to ensure positioning tolerances are met during operation, while minimally affecting the disc drive""s storage capacity. To accomplish this task, each of the three types of write failures are individually addressed, and a system for measuring and establishing parameters that characterize the recording head position responsible for each mode of failure is utilized.
More specifically, a rotational shock that would cause an adjacent data sector to be overwritten can be detected by measuring the amount of time that elapses from the moment the bead passes over the sector address mark (SAM) of adjacent sectors. Given the known rotational velocity of the disc, and rotational distance between adjacent servo sectors, the disc drive can calculate the expected time delay between SAMs. If the delay is outside tolerance, the disc drive can predict an adjacent sector overwrite. A rotational shock that would cause a write fault can be detected by interpreting the PES in the servo sectors. If the disc drive detects an incorrect position error signal (PES), an off-track write fault can be predicted. A translational shock that would cause a skip write can be detected by comparing the amplitude of the PES signal to an expected value. If the recording head moves too far from the disc, the amplitude would be outside of tolerance, and a skip write can be predicted.
The number of servo sectors on a disc must be increased to acquire enough parametric data to accurately interpret the above measurements. However, increasing the number of servo sectors results in a substantial amount of non-user data on the disc, decreasing the disc drive""s storage capacity. A method in accordance with one preferred embodiment of the invention compensates for this by decreasing the size (length) of the servo sector format needed. Finally, the cumulative effect of the unused reader to writer gap space resulting from the piggy-back recording head configuration for each of the increased number of servo sectors becomes a predominant factor for unused storage space on the disc and thus a revised recording head configuration is provided to remove the write gap and thus regain usable disc real estate for data storage.
An embodiment of the invention utilizes a side-by-side read/write element configuration to eliminate this gap. In addition, a large number of micro-servo sectors recorded on the disc surfaces provides frequent position information to the control system to enable detection of track mis-registrations and shock events. Essentially, a disc is formatted with a large number of small servo sectors. The servo sectors further comprise a unique format by which track and disc location is stored. A unique side-by-side recording head is utilized in conjunction with the small servo sectors. The disc drive takes, through the recording head passing over the numerous servo sectors, frequent measurements that can be used to characterize the position of the recording head in reference to the disc surface. The disc drive therefore, in effect, without affecting capacity or performance, verifies that data is likely to be correctly written to the disc. These and various other features as well as advantages that characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.